This present invention relates to catalytic electrodes and production process thereof, and also to electrochemical devices and fabrication process thereof.
As electrolytes for solid polymer fuel cells, perfluorosulfonate ionomers (PFSIs) led by “Nafion” (product of E. I. DuPont de Nemours & Co.) have been used conventionally. Employed, for example, as diaphragms for permitting conduction of protons between an anode and a cathode while preventing direct mixing of reactant gases include those obtained by dissolving PFSI in an alcohol (e.g., ethanol or isopropanol) or the like and then conducting casting, drying, and heating treatment; and those obtained by directly processing a resin, in which intramolecular sulfonate groups are bonded with cations such as sodium ions, into a sheet form and then substituting the cations with hydrogen atoms.
In a catalyst layer where an actual reaction is conducted, it is important to form a three-phase interface at which gases, a solid electrolyte, and a catalyst are brought into contact with each other. Therefore, the catalyst layer is formed, for example, by mixing a PFSI-dissolved solution with the catalyst and coating the resultant mixture directly on an electrolyte membrane or coating it on a “TEFLON” (trademark), (polytetrafluoroethylene (PTFE)) sheet, or the like and then transferring the resultant coating layer onto an electrolyte membrane.
In MEA (Membrane and Electrode Assembly) formed by using PFSI as described above, the durabilities of the ion conductor and catalyst layer are dependent upon the durability of PFSI.
In a fuel cell formed using PFSI as mentioned above, however, PFSI dissolves in alcohol or water as a solvent. PFSI, therefore, was caused to swell or was dissolved in certain instances not only during long-time operations but also even during short-time operations when the operations were performed under high-temperature conditions or methanol was used as a fuel. As a result, PFSI was caused to separate from or to flow out of MEA, which makes up the fuel cell, so that impairments in characteristics were observed. In particular, any flowing-out or separation of PFSI inside the catalyst layer reduces the reaction area, and hence, leads to substantial impairments in characteristics.
In the meantime, processes for forming electrodes for fuel cells from catalysts and polytetrafluoroethylene (PTFE) have been proposed, for example, in Japanese Patent Laid-open No. 2001-57216 as a production process of electrodes for phosphoric acid fuel cells.
FIG. 4 is a simplified schematic cross-sectional view of a conventional electrode formed from a catalyst and polytetrafluoroethylene (PTFE). As depicted in FIG. 4, an electrode 11 is composed of a current collector 13 and a catalyst layer 12 formed on the current collector 13, and the catalyst layer 12 is formed from PTFE 14 and catalyst particles 15 such as platinum particles.
The conventional electrode 11, which is provided with the catalyst layer 12 formed of PTFE 14 and the catalyst particles 15, however, still requires improvements in the assurance of internal ion-conducting paths, mechanical strength, and thermal stability of the catalyst layer 12.